Background Art—I
In a conventional active matrix type liquid crystal display device employing amorphous silicon (hereafter referred to as “a-Si”) transistors, a-Si has been considered sufficient to meet the performance requirements for driving the pixels. However, using a-Si, integrating a signal driver circuit on the very substrate on which the pixels are formed is difficult because of the characteristics of a-Si. For this reason, the display panel is usually driven by an external driver circuit (also simply referred to as a driver) formed by using single crystal Si.
In such a device, it is necessary to connect IC chips in the driver to the array substrate. An example of a technique for the connecting is the tape carrier package (TCP), as shown in FIG. 31, in which a driver 302 is mounted on a tape carrier film 301 so as to be connected to an array substrate 303 of the liquid crystal display panel.
In order to achieve a reduction in the thickness and weight of the device, the chip on glass (COG) technique, in which a driver is directly mounted on a liquid crystal panel, has been suggested. This technique eliminates the tape carriers and accordingly achieves cost reduction. In addition, the total number of the connections in a liquid crystal panel, including the connections with drivers, is reduced to ⅓ to ⅕ of that in TCP, resulting in an increase in reliability. This technique is illustrated in FIG. 32.
Although the number of connections of the driver IC chips is smaller in COG than that in TCP, COG also requires a high precision mounting process to connect a large number of terminals, which makes it difficult to achieve a significant increase in reliability and a remarkable reduction in manufacturing costs.
Unlike liquid crystal display devices using amorphous silicon TFTs for the switching elements of the active matrix, liquid crystal display devices employing polysilicon TFTs (hereafter referred to as p-Si TFTs) exhibit a mobility of the semiconductor layer at least 10 times to 100 times higher than that of a-Si (see SID 97, p. 171) and therefore make it possible to integrally form both active matrix elements for the display and portions of or all of signal driver circuits on a glass substrate at one time.
The driver circuit comprises, for example, shift registers, latches, and so forth, formed by a multiplicity of CMOS (Complimentary Metal Oxide Semiconductor) inverters, each composed of a p-channel TFT 304 and an n-channel TFT 305 as shown in FIG. 33. The wiring lines connecting the p-channel TFTs 304, the power supply lines, and the image signal lines are composed of, for example, an aluminum thin film that is formed on the glass substrate and has a thickness of about 7000 Å.
In the prior art liquid crystal display device, owing to the characteristics of p-Si TFT and line resistance of the power supply line, a voltage drop occurs in the power supply voltage, which is supplied to the shift resisters and so forth. Therefore, the prior art liquid crystal display device has such a drawback that unless the width of the power supply line is made fairly large or the power source voltage is made fairly high, the driver circuit does not properly operate.
As mentioned above, p-Si TFTs can achieve a higher operation speed than a-Si TFTs. However, as shown in, for example, “Integrated driver circuits for active matrix liquid crystal displays,” Displays Vol. 14, No. 2, 1993, pp. 104–114 (see FIGS. 34(a) and 34(b)), p-Si TFTs have a larger OFF current and a larger subthreshold region current than those in single crystal silicon transistors, which are generally used in IC chips. This is considered to be due to the hopping of carriers between grain boundary levels in polysilicon or due to the influence of fixed charge caused by ions present in the gate insulating layer (Memorandum No. UCB/ERL M93/82). For this reason, in a switching operation of the CMOS inverter, as a drain current increases in the subthreshold region, a larger shoot-through current occurs.
Referring now to FIG. 35, more specific details of the shoot-through current are illustrated below.
(1) When an input voltage (gate voltage) Vin is 0 V, the p-channel TFT 304 is in an ON state and the n-channel TFT 305 is in an OFF state, and the output voltage Vout becomes a high level (5V=Vdd). In this state, substantially no shoot-through current (DC path current) flows from the source of the p-channel TFT 304 to the drain of the n-channel TFT 305.
(2) During the period in which the input voltage Vin increases and exceeds a threshold voltage Vth(n) (voltage A) of the n-channel TFT 305 and reaches a voltage B, the p-channel TFT 304 maintains the ON state in a saturation region, while the n-channel TFT 305 is in a non-saturation region and a drain current corresponding to the input voltage Vin starts to flow. Accordingly, the shoot-through current gradually increases, while the output voltage Vout gradually decreases.
(3) During the period in which the input voltage Vin further increases from the voltage B and reaches a voltage D, both p-channel TFT 304 and n-channel TFT 305 are in a non-saturation region and a drain current corresponding to the input voltage Vin flows therein. Accordingly, the shoot-through current reaches the maximum value at a voltage C, and the output voltage Vout shows a sudden drop.
(4) When the input voltage Vin exceeds a voltage D, the p-channel TFT 304 is in a non-saturation region and a drain current corresponding to the input voltage Vin flows therein, while the n-channel TFT 5 is in a non-saturation region and substantially in a ON state. Accordingly, the shoot-through current reduces, and the output voltage Vout asymptotically approaches a low level (0 V).
(5) When the input voltage Vin exceeds the threshold voltage Vth(p) (voltage E), the p-channel TFT 304 is turned to an OFF state and the n-channel TFT 305 an ON state. Accordingly, the output voltage Vout becomes the low level (0 V) and substantially no shoot-through current flows therein.
Since such a shoot-through current occurs as described above, when the amount of the voltage drop caused by the line resistance of the power supply lines becomes 1.5 V or greater, margins of driving voltage for shift registers and latches become exceedingly small, making the driver circuits difficult to properly operate. Specifically, when a liquid crystal display device being 20 cm in diagonal size is required, a current of about 160 mA flows in a power supply line thereof In such a device, in order to control the voltage drop within 1.5 V or smaller, it is required that the wiring line resistance of the power supply line be about 9 Ω or lower. Therefore, when the sheet resistance of the power supply line is 0.1 Ω, the width of the wiring line needs to be 3.4 mm or larger per line, in order to ensure proper operation of the driver circuits.
Such a problem is more serious in the cases of liquid crystal display devices having a large number of display pixels and liquid crystal display devices capable of color display, since in these devices, the number of stages of shift registers is large and the voltage drop in the power supply voltage is accordingly large. Furthermore, although such a problem also arises both in liquid crystal display devices operated by analog image input signals and in those operated by digital image input signals, but especially the latter devices are more susceptible to the problem since they have, in addition to the shift registers, latch circuits and D-A converters corresponding to the bit number of the digital image signals and the shoot-through current is correspondingly large.
The above-described problem also exists in a so-called point-at-a-time driving type liquid crystal display device, in which image signal voltage is sequentially applied to the pixel electrodes, such as shown in Japanese Examined Patent Publication No. 4-3552, and in a line-at-a-time driving type liquid crystal display device, in which image signals for one horizontal period is retained and thereafter image signal voltages are applied to the pixel electrodes of the horizontal line at one time, such as shown in SID '96 Digest pp. 21–24.
Background Art-II
At present, liquid crystal display devices are widely used in such appliances as notebook computers and automobile navigation systems, and in the devices, further reduction in size and thickness is desired. In order to achieve the size and thickness reduction, the use of polycrystalline silicon thin film transistors, which makes it possible to integrate driver circuits in the array substrate, is considered effective to simplify the manner of connection of the driver circuits with external circuits.
Accordingly, in the following discussion referring to drawings, there are described a prior art device in which amorphous silicon thin film transistors are connected to driver ICs for driving the transistors by using flip chip technique and a prior-art connection technique for connecting a prior art device in which polycrystalline silicon thin film transistors are used to external circuits.
FIGS. 36 and 37 schematically show the configurations of a 5-inch liquid crystal display device having about 400 thousand pixels. FIG. 36 shows a plan view of a liquid crystal display device in which prior art amorphous silicon thin film transistors are connected to driver ICs by using a flip chip technique, and a cross-sectional view of the device taken along the line A–A′ in the plan view. FIGS. 37(A) and 37(B) show a plan view of a liquid crystal display device in which the driver circuit is made of a polycrystalline silicon thin film and a cross-sectional view of the device taken along the line B–B″.
In FIGS. 36(A), 36(B), 37(A), and 37(B), like parts are designated by like reference numerals. Reference numeral 401 indicates an array substrate, reference numeral 402 a counter substrate, reference numeral 402 a flexible wiring board, and reference numeral 411 a driver IC.
As shown in FIGS. 36(A) and 36(B), in a device in which ICs are connected using a flip chip technique, if an additional flexible wiring board were not provided, the pitches of the connections would be so small that they would be beyond the current state of the art, and therefore, flexible wiring boards are provided on opposing sides and are connected to a printed circuit board (not shown) to construct circuitry.
The device shown in FIGS. 37(A) and 37(B) has a driver circuit section formed of a polycrystalline silicon thin film. Unlike the prior art amorphous silicon thin film transistors, the signal circuit section can be formed on one side, and therefore only one flexible wiring board is required for the connection with the printed wiring board to construct the circuitry.
As described above, the prior art method in which amorphous silicon thin film transistors and driver ICs are connected using a flip chip technique requires two flexible wiring boards, which increases device costs, and moreover, since the device has such a configuration that both sides of the flexible wiring boards are connected by a printed wiring board disposed on the side of the backlight, the liquid crystal device becomes bulkier.
When the driver circuits are formed using polysilicon thin film transistors, the flexible wiring board is necessary only for one side because there are no restrictions in the connection pitches and accordingly cost reduction is possible to a certain degree. However, the flexible wiring board must be connected to a relatively large shaped printed circuit board, which requires that the printed wiring board be disposed on the backside of the device, and the problem that the liquid crystal device is bulky remains unsolved.